State-of-the-art lithium-ion cells have a lithiated carbon negative electrode, or anode, (LixC6) and a lithium-cobalt-oxide positive electrode, or cathode, Li1−xCoO2. During charge and discharge of the cells, lithium ions are transported between the two host structures of the anode and cathode with the simultaneous oxidation or reduction of the host electrodes, respectively. When graphite is used as the anode, the voltage of the cell is approximately 4 V. The LiCoO2 cathode, which has a layered structure, is expensive and becomes unstable at low lithium content, i.e., when cells reach an overcharged state at x>0.5. Alternative, less expensive electrode materials that are isostructural with LiCoO2, such as LiNi0.8Co0.2O2, LiNi0.5Mn0.5O2 and LiMn0.33Ni0.33CO0.33O2 are being developed with the hope of replacing at least part of the cobalt component of the electrode. However, these layered structures, when extensively delithiated become unstable, because of the high oxygen activity at the surface of the particles. Therefore, the delithiated electrode particles tend to react with the organic solvents of the electrolyte or lose oxygen. Such reactions at the surface of metal-oxide- and lithium-metal-oxide electrodes, in general, are detrimental to the performance of the lithium cells and batteries, and methods are required to combat these reactions to ensure that maximum capacity and cycle life can be obtained from the cells.
Several efforts have already been made in the past to overcome the stability and solubility problems associated with lithium-metal-oxide electrodes. For example, considerable success has been achieved by stabilizing electrodes by pre-treating the electrode powders with oxide additives such as Al2O3 or ZrO2 obtained from metal alkoxide precursors such as solutions containing aluminum ethylhexanoate diisopropoxide (Al(OOC8H15)(OC3H7)2 or zirconium ethylhexanoisopropoxide (Zr[(OOC8H15)2(OCH3H7)2]) as described, for example, by J. Cho et al in Chemistry of Materials, Volume 12, page 3788 (2000) and J. Cho et al in Electrochemical and Solid State Letters, Volume 4 No. 10, page A159 (2001), respectively, or a zirconium oxide, polymeric precursor or zirconium oxynitrate (ZrO(NO3)2.xH2O) as described by Z. Chen et al in Electrochemical and Solid State Letters, Volume 5, No. 10, page A213 (2002), prior to the fabrication of the final electrode thereby making the surface of the LiCoO2 particles more resistant to electrolyte attack, cobalt dissolution or oxygen loss effects.
The loss of oxygen from lithium metal oxide electrodes, such as layered LiCoO2 and LiNi1−yCoyO2 electrodes can contribute to exothermic reactions with the electrolyte and with the lithiated carbon negative electrode, and subsequently to thermal runaway if the temperature of the cell reaches a critical value. Although some success has been achieved in the past to improve the performance of lithium-ion cells by coating electrode particles, the coatings can themselves impede lithium diffusion in and out of the layered electrode structure during electrochemical discharge and charge. Further improvements in the composition of high potential metal-oxide- and lithium-metal oxide electrodes, particularly at the surface of the electrodes, and in methods to manufacture them are still required to improve the overall performance and safety of lithium cells.
Lithium metal oxides that have a spinel-type structure are alternative electrodes for commercial lithium-ion cells and batteries, notably those used in high-power applications. Of particular significance is the lithium-manganese-oxide spinel, LiMn2O4, and its cation-substituted derivatives, LiMn2−xMxO4, in which M is one or more metal ions typically a monovalent or a multivalent cation such as Li+, Mg2+ and Al3+, as reported by Gummow et al. in U.S. Pat. No. 5,316,877 and in Solid State Ionics, Volume 69, page 59 (1994). It is well known that LiMn2O4 and LiMn2−xMxO4 spinel electrodes are chemically unstable in a lithium-ion cell environment, particularly at high potentials and/or when the cell operating temperature is raised above room temperature, when manganese ions from the spinel electrodes tend to dissolve in the electrolyte. This process is believed to contribute to the capacity loss of the cells at elevated temperatures. Moreover, the removal of all the lithium from LiMn2O4 and LiMn2−xMxO4 electrodes yields a MnO2 component, which itself is a strong oxidizing agent. The surface of such delithiated spinel electrodes can have a high oxygen activity, thereby possibly inducing unwanted oxidation reactions with the electrolyte. Although considerable progress has been made to suppress the solubility and high-temperature performance of spinel electrodes and to improve their stability by cation doping, as described for example by Gummow et al. in U.S. Pat. No. 5,316,877, or by forming oxyfluoride compounds as described by Amatucci et al. in the Journal of the Electrochemical Society, Volume 149, page K31 (2002) and by Choi et al. in Electrochemical and Solid-State Letters, Volume 9, page A245-A248 (2006), or by surface coatings as described by Kim et al. in the Journal of the Electrochemical Society, Volume 151, page A1755 (2004), these treatments have not yet entirely overcome the cycling instability of cells containing manganese-based spinel electrodes.
Furthermore, other metal-oxide- and lithium-metal-oxide electrode materials that are good oxidants are of interest for lithium batteries are known, for example, V2O5, and materials containing a V2O5 component, such as LiV3O8 and AgV3O8, that can be written alternatively in two-component notation as Li2O.3V2O5 and Ag2O.V2O5, respectively, and Ag2V4O11 that can be written alternatively in two-component notation as Ag2O.2V2O5. The silver-containing materials, notably Ag2V4O11, are of particular interest for primary lithium cells in medical devices such as cardiac defibrillators. In this case, a preconditioned electrode with a stable surface layer will help prolong the life of the cell, particularly if left standing in the charged state or partially charged state for long periods of time. The invention extends to include MnO2 and MnO2-containing compounds which, like V2O5, are strong oxidants, such as Li2O.xMnO2 and Ag2O.xMnO2 (x>0) electrode compounds.
It is clear from the prior art that further advances are required, in general, to improve the surface stability of metal-oxide and lithium-metal-oxide electrodes for non-aqueous lithium cells and batteries. This invention relates to such improvements, notably those that are achieved from stabilized electrode surfaces that are engineered by preconditioning electrode particles with aqueous or, preferably, non-aqueous solutions in which the dissolved salts contain both stabilizing cations and anions. The invention relates more specifically to uncycled, preconditioned metal oxide- or lithium metal oxide electrodes, the electrodes being preconditioned in an aqueous or a non-aqueous solution containing stabilizing cations and anions, such that the stabilizing ions are etched into the electrode surface to form a protective layer. Methods of preconditioning the electrodes are disclosed as are electrochemical cells and batteries containing the electrodes.